1. Field of the Invention
The present invention relates to a multilayer insulated wire having three or more insulating layers and a manufacturing method therefor, and more particularly, to a multilayer insulated wire, which enjoys a high coilability and is adapted for use as a winding or lead wire of a transformer incorporated in electrical or electronic equipment, and in which separability between insulating layers is so good that the insulating layers can be removed, and solder is allowed to adhere to a conductor in a short period of time when they are dipped in a solder bath, so that the solderability is high, further the insulation properties of the insulating layers cannot be easily lowered with time, and a manufacturing method for the multilayer insulated wire.
2. Prior Art
The construction of a transformer is prescribed by IEC (International Electrotechnical Commission) standards Pub. 950, 65, 335, 601, etc. These standards provide that an enamel film which covers a conductor of a winding be not authorized as an insulating layer, and that at least three insulating layers be formed between primary and secondary windings or the thickness of an insulating layer be 0.4 mm or more. The standards also provide that the creeping distance between the primary and secondary windings, which varies depending on the applied voltage, be 5 mm or more, that the transformer withstand a voltage of 3,000 V applied between the primary and secondary sides for a minute or more, and the like.
Accordingly, a currently prevailing transformer has a profile such as the one illustrated in FIG. 1. Referring to FIG. 1, a flanged bobbin 2 is fitted on a ferrite core 1, and an enameled primary winding 4 is wound around the bobbin 2 in a manner such that insulating barriers 3 for securing the creeping distance are arranged individually on the opposite sides of the peripheral surface of the bobbin. An insulating tape 5 is wound for at least three turns on the primary winding 4, additional insulating barriers 3 for securing the creeping distance are arranged on the insulating tape, and an enameled secondary winding 6 is then wound around the insulating tape.
Recently, a transformer which includes neither the insulating barriers 3 nor the insulating tape 5, as shown in FIG. 2, has started to be used in place of the transformer having the profile shown in FIG. 1.
The transformer shown in FIG. 2 has an advantage over the one shown in FIG. 1 in being able to be reduced in overall size and dispense with the winding operation for the insulating tape.
In manufacturing the transformer shown in FIG. 2, it is necessary, in consideration of the aforesaid IEC standards, that at least three insulating layers 4b (6b), 4c (6c), and 4d (6d) are formed on one or both of conductors 4a and 6a of primary and secondary windings 4 and 6 used, and that the individual insulating layers can be separated from one another.
One such known winding is described in Jpn. UM Appln. KOKAI Publication No. 3-106626. In this case, an insulating tape is first wound around a conductor to form a first insulating layer thereon, and is further wound to form second and third insulating layers in succession. Thus, three insulating layers are formed so as to be separable from one another. In another known winding disclosed in Jpn. UM Appln. KOKAI Publication No. 3-56112, a conductor enameled with polyurethane is successively extrusion-coated with fluoroplastics, whereby extrusion-coating layers composed of three layers structure are formed for use as insulating layers.
In the former case, however, winding the insulating tape is an unavoidable operation, so that the efficiency of production is extremely low, thus entailing increased manufacturing cost.
In the latter case, the insulating layers, which are formed of fluoroplastics, enjoy a satisfactory thermal resistance. Since the adhesion between the conductor and the insulating layers and between the insulating layers is poor, however, the resulting insulated wire lacks in reliability.
In coiling the insulated wire, it is guided through a guide nozzle as it is wound around a coil bobbin. During this operation, the insulating layers may be easily separated from the conductor as the insulated wire rubs against the guide nozzle, or the insulating layers may be separated from one another. If the wire in this state is wound around the coil bobbin, the insulating layers are torn by the friction between the adjacent turns of the insulated wire or the like. In this situation, the electrical properties, e.g., dielectric breakdown properties, of the resulting coil are spoiled.
The insulating layers cannot be removed by being dipped into a solder bath. In processing terminals for the connection between the insulated wire and lead pins, for example, therefore, the insulating layers at the terminals must be removed by some low-reliability mechanical means.
In order to solve such problems as aforesaid, an investigation is being made into an arrangement such that a conductor is extrusion-coated with a polyethylene terephthalate (PET) resin, which enjoys high electrical insulation properties and thermal resistance and easily decomposes at the melting temperature of solder, to form an insulating layer.
However, this PET resin cannot fulfill its proper thermal resistance and mechanical properties until it is crystallized under appropriate conditions which make that resin orientate. Therefore, a highly crystallized insulating layer cannot be obtained by extrusion-coating, so that the dielectric strength requires improvement.
In the case of a wire in which all three insulating layers are formed of the PET resin, there is room for improvement in the insulation properties of a coil formed by coiling the wire.
This problem may be attributable to the following circumstances. Since the surface of each PET resin layer, formed as an insulating layer, has a high coefficient of friction, the insulating layers are liable to cracking or damages as they rub against the guide nozzle of a coiling machine during coiling operation. Moreover, the adhesion between the insulating layers, formed the PET resin, is so good that cracks and the like in the outermost layer easily affect the lower insulating layers due to a notch effect.
Also known is an insulated wire, though not multilayer, in which a bondable layer is formed as the outermost layer by coating a resin such as polyamide on the surface of an enameled wire by baking.
As this insulated wire is wound into a coil, its increasing turns are adhered together, so that the coil can be prevented from loosening. Thus, the reliability of the coil as a final product can be improved, and the efficiency of coil production can be increased.
Usually, the bondable layer of the above-described insulated wire is formed by applying a paint, which is composed of a bondable resin dissolved in a solvent, to the surface of an enameled wire and then baking the resulting structure. Accordingly, the wettability of the interface between the bondable layer and an insulating film covering the enameled wire is improved, so that the bondable layer can firmly adhere to the insulating film with ease. Thus, various materials can be utilized for the bondable layer.
If a multilayer insulated wire, like the insulated wire described above, is formed having the bondable layer outside the triple insulating layers, the resulting coil can be prevented from loosening by the high-bonding strength of the bondable layer during the coiling operation, and the reliability of the coiling operation can be improved.
However, no solvent is used for the manufacturing of the bondable multilayer insulated wire in which the individual insulating layers and the bondable layer on the outermost insulating layer are formed by extrusion-coating. Unlike the enameled wire, therefore, this insulated wire cannot enjoy the effect of the solvent to improve the wettability of the interface between the bondable layer and the outermost insulating layer positioned under the bondable layer.
Accordingly, the force of adhesion between the bondable layer and the outermost insulating layer cannot be very great.
When the bondable multilayer insulated wire is coiled, therefore, the outside bondable layer sometimes may be separated from the insulating layer thereunder or scraped off by friction with the guide nozzle. Thus, even if the bondable layer remains on the outermost insulating layer, its adhesiveness is lowered considerably.
In the case of a multilayer insulated wire which complies with the aforementioned IEC standards, interlaminar separation between at least three insulating layers is in the state of being possible. If the bondable layer, the outermost layer, is separated or scraped off and adheres to the inner surface of the guide nozzle, therefore, the following awkward situations are liable to be entailed.
First, a tension which acts on the insulated wire being wound increases, so that snapping of the wire is caused between the guide nozzle and the coil bobbin. Further, the constituent resin of the bondable layer adhering to the inner surface of the guide nozzle rubs against the insulating layers, thereby tearing the insulating layers, and moreover, causing the insulating layers to be separated from one another. If the insulated wire is wound around the coil bobbin in this state, the insulating layers are torn by the friction between the adjacent turns of the wound wire.
If the insulating layers are torn in this manner, the electrical insulation properties, e.g., dielectric breakdown properties, of the coil are ruined.